3.7 How make Reads a Makefile

GNU make does its work in two distinct phases. During the first
phase it reads all the makefiles, included makefiles, etc. and
internalizes all the variables and their values, implicit and explicit
rules, and constructs a dependency graph of all the targets and their
prerequisites. During the second phase, make uses these internal
structures to determine what targets will need to be rebuilt and to
invoke the rules necessary to do so.

It’s important to understand this two-phase approach because it has a
direct impact on how variable and function expansion happens; this is
often a source of some confusion when writing makefiles. Here we will
present a summary of the phases in which expansion happens for different
constructs within the makefile. We say that expansion is
immediate if it happens during the first phase: in this case
make will expand any variables or functions in that section of a
construct as the makefile is parsed. We say that expansion is
deferred if expansion is not performed immediately. Expansion of
a deferred construct is not performed until either the construct appears
later in an immediate context, or until the second phase.

You may not be familiar with some of these constructs yet. You can
reference this section as you become familiar with them, in later
chapters.

For the append operator, ‘+=’, the right-hand side is considered
immediate if the variable was previously set as a simple variable
(‘:=’ or ‘::=’), and deferred otherwise.

For the shell assignment operator, ‘!=’, the right-hand side is
evaluated immediately and handed to the shell. The result is stored in the
variable named on the left, and that variable becomes a simple variable
(and will thus be re-evaluated on each reference).

Conditional Directives

Conditional directives are parsed immediately. This means, for
example, that automatic variables cannot be used in conditional
directives, as automatic variables are not set until the recipe for
that rule is invoked. If you need to use automatic variables in a
conditional directive you must move the condition into the
recipe and use shell conditional syntax instead.

Rule Definition

A rule is always expanded the same way, regardless of the form:

immediate : immediate ; deferreddeferred

That is, the target and prerequisite sections are expanded immediately,
and the recipe used to construct the target is always deferred. This
general rule is true for explicit rules, pattern rules, suffix rules,
static pattern rules, and simple prerequisite definitions.